Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification

Generally, the electron-withdrawing substitution on the end-capping group of the acceptor-donor-acceptor type small-molecule acceptor (SMA) narrows the optical bandgap, and the electron-donating group lifts the lowest unoccupied molecular orbital (LUMO) energy level of nonfullerene SMA, which increase the short-circuit current density (J _SC ) and open circuit voltage (V _OC ) of the organic solar cells (OSCs), respectively; however, their synergistic effect on the properties of SMA has remained elusive. Here, we first report a new end-capping group (EG), namely, 5-fluoro-6-methyl-3-dicycanovinylindan-1-one (CFDCI), that concurrently possesses an electron-withdrawing fluorine substitute and an electron-donating methyl group. A prototype SMA (namely, ITCF) based on CFDCI and its two control counterparts were prepared to fully understand the structure-property relationship that the new EG exerts on the resultant SMA. The ITCF demonstrated a moderately crystalline morphology in pristine film and more balanced charge transport properties as well as a reduced amount of bimolecular recombination in blend film in comparison with its counterparts. The ITCF-based devices demonstrated a high power conversion efficiency (PCE) of 13.25% with an outstanding fill factor (FF) of 78.8%, which significantly outperformed their counterparts. Our study provides an important strategy to judiciously tune the properties of the SMAs for improving the performance of the OSCs.